As a wiring substrate used for electronic devices, a glass ceramic substrate constituted of a sintered compact of a composition containing glass powder and ceramic powder is known. A conductive pattern is formed, for example, on a surface of or inside the glass ceramic substrate, and the substrate is mounted as a wiring substrate in an electronic device to be used. Alternatively, the glass ceramic substrate sometimes has no wiring provided thereon in particular to be used as a housing for electronic device such as a portable phone.
In recent years, accompanying size reduction and increase in function of electronic devices, thinning of glass ceramic substrate is also demanded. Moreover, electrode structures are becoming complicated accompanying complication and micronization of circuit boards, and stress applied to a glass ceramic substrate is also increasing. Accordingly, a glass ceramic substrate having a higher strength than conventional ones is demanded now. Further, when it is used for an element mounting substrate for LED or a housing for electronic device or the like, a glass ceramic substrate having a sufficient strength and also having a three-dimensional shape is demanded.
Here, glass and ceramic as a main component of the glass ceramic substrate are both brittle materials and the glass ceramic substrate has a nature that it is inherently weak against shock and a crack easily occurs. Accordingly, attempts have been made hitherto to obtain a glass ceramic substrate that can correspond to thinning and high strength, by selecting one that can contribute to improvement in strength of a glass ceramic substrate as a compounded ceramic powder, or the like.
For example, for the purpose of increasing heat conductivity and increasing strength of the glass ceramic substrate, Patent Reference 1(JP-A 2002-111210) proposes a glass ceramic substrate in which flat ceramic particles having an aspect ratio of 4 or more are dispersed in a glass matrix with a high orientation degree of 50% or more. Here, in the glass ceramic substrate proposed by Patent Reference 1, by using flat ceramic particles, strength property is improved compared to conventional ones. However, we are in a situation where it cannot correspond to high strength property demanded in recent years.
Further, Patent Reference 2(JP-A H06-029664) describes a low temperature fired multilayered ceramic wiring board composed of glass and crystalline material being a residual part, in which a thermal expansion coefficient of its outermost layer is made smaller than that of its inner layer and the total thickness of its outermost surface layer and its outermost rear layer is made smaller than the thickness of the inner layer. By employing such a structure, in a cooling process after firing, compressive stress is generated in the outermost surface layer and the outermost rear layer, so that transverse rupture strength of the multilayered ceramic wiring board improves.
Further, Patent Reference 3(JP-B 4957723) describes a multilayer ceramic substrate having a laminated structure including surface layer portions and an inner layer portion, which uses the concept similar to that of Patent Reference 2. In this substrate, a thermal expansion coefficient of the surface layer portions is smaller than that of the inner layer portion, the difference in thermal expansion coefficient between the surface layer portions and the inner layer portion is 1.0 ppm/K or more, and the weight content of a component common to both a material constituting the surface layer portions and a material constituting the inner layer portion is 75% by weight or more. By employing such a structure, defects such as delamination and voids in an interface portion between the surface layer portion and the inner layer portion and warpage of the substrate are suppressed and substrate strength improves.
However, in Patent Reference 3, in order to improve mechanical strength, a crystallized glass composition in which crystals precipitate as a glass composition is used, and in order to suppress delamination in the interface portion between the surface layer portion and the inner layer portion, a large amount of component common to both the constituting materials needs to be contained. Further, since the crystallized glass composition is used, a dispersion in strength increases because of an amount of crystal to precipitate, a size of crystallite, or the like and firing has to be performed at 980° C., and thus there is caused a problem that silver as a conductor wiring cannot be used.
Further, in Patent References 2 and 3, compressive stress is applied to the outer layer portion, but mechanical strength in the inner layer portion to which tensile stress is applied is weak, so that we are in a situation where Patent References 2 and 3 cannot correspond to high strength property demanded in recent years.